1. Field of the Invention
The present invention relates to a magnetic detecting apparatus that employs a giant magnetoresistive element to issue a pulse signal in response to the movement of a magnetic object, e.g., the number of revolutions of a magnetic rotating body and, more particularly, to a magnetic detecting apparatus that prevents malfunctions in a waveform processing circuit attributable to level fluctuations, superimposed noises, etc. in the analog signals from the giant magnetoresistive element, thereby achieving higher accuracy and reliability.
2. Description of Related Art
There is well known a magnetic detecting apparatus for detecting the rotation of a magnetic rotating body such as the crankshaft of an internal-combustion engine. It is also widely known that a giant magnetoresistive element, which exhibits high detection sensitivity, is used as a magnetic detecting head.
This type of magnetic detecting apparatus is equipped with a waveform processing circuit for waveform-shaping the analog signals issued from the giant magnetoresistive element into pulse signals so as to output pulse signals based on the number of revolutions.
FIG. 4 is a side view illustrative of a magnetic detecting apparatus, e.g., a rotation detecting apparatus, which incorporates a typical waveform processing circuit; it outlines the relative position relationship between a magnetic body 100 which rotates in the direction of arrow and a rotation detecting apparatus 101.
In FIG. 4, the magnetic body 100 to be detected is provided as an integral part of a rotating shaft such as a crankshaft; it has projected sections 100a and recessed sections 100b formed alternately on the outer periphery thereof. The rotation detecting apparatus 101 is disposed so that it is opposed to the magnetic body 100 with a predetermined gap G therebetween.
FIG. 5 is a sectional view outlining the structure of the rotation detecting apparatus 101 in FIG. 4.
In FIG. 5, the rotation detecting apparatus 101 is equipped with a magnetic detection head which is constituted by a permanent magnet 103 and an IC 104 and which is disposed on the surface facing against the magnetic body 100.
The IC 104 incorporates a giant magnetoresistive element and a waveform processing circuit (which will be discussed later) which are combined into one piece; the giant magnetoresistive element is disposed to be opposed to the magnetic body 100, and the waveform processing circuit converts the analog signals received from the giant magnetoresistive element into pulse signals and outputs the pulse signals.
The permanent magnet 103 is disposed at the rear of the IC 104 to apply a bias magnetic field to the giant magnetoresistive element in the IC 104.
FIG. 6 is a perspective view illustrative of the relative position relationship between the magnetic body 100 and the rotation detecting apparatus 101 shown in FIG. 4.
In FIG. 6, a giant magnetoresistive element 20 placed on the IC 104 is composed of line-shaped segments 20a through 20d which are divided into two pairs and which are provided to improve the magnetic detection sensitivity.
Of the segments 20a through 20d, one pair of the segments 20a and 20b are positioned on the front with respect to the direction of the rotation of the magnetic body 100 (see the arrow), while the other pair of the segments 20c and 20d are positioned on the rear with respect to the direction of the rotation of the magnetic body 100.
FIG. 7 is a circuit diagram showing the connection of the respective segments 20a through 20d constituting the giant magnetoresistive element 20.
In FIG. 7, the segments 20a through 20d constitute two pairs of series circuits to include the segments of each of the aforesaid two pairs; the respective series circuits are connected in parallel between a power supply Vcc and ground to form a bridge circuit 14.
A midpoint 14a of one series circuit which include the segments 20a and 20d in the bridge circuit 14 and a midpoint 14b of the other series circuit which includes the segments 20c and 20b provide output terminals through which analog signals A1 and A2 are output as the magnetic body 100 rotates.
FIG. 8 shows the curves illustrative of the resistance values in ohms (.OMEGA.) of the giant magnetoresistive element 20 which change as the field magnetic in oersteds (Oe) changes.
In FIG. 8, the resistance value of the giant magnetoresistive element 20 markedly changes in comparison with a regular magnetoresistive element when the external magnetic field changes; for instance, it increases or decreases approximately 250 .OMEGA., i.e. 10% or more, for a resistance value of about 2000 .OMEGA., and it exhibits hysteresis in relation to the changing direction of the magnetic field.
The aforesaid giant magnetoresistive element 20 is described, for example, in "Magnetoresistance Effect of Synthetic Lattice" (page 813 through page 821, No. 51991, Vol. 15, Japan Applied Magnet Society Journal).
According to the foregoing literature, the giant magnetoresistive element 20 can be constructed by a laminated body known as a synthetic lattice film, composed by alternately stacking magnetic layers and nonmagnetic layers that are a few angstroms to a few tens of angstroms (.ANG.).
The giant magnetoresistive element 20 employs, as its material, (Fe/Cr)n, (permalloy/Cu/Co/Cu)n, (Co/Cu)n, etc.
FIG. 9 and FIG. 10 are explanatory diagrams illustrative of the relationship between the magnetic body 100 and the waveforms of the analog signals A1 and A2; FIG. 9 shows a case wherein the gap G between the magnetic body 100 and the magnetic detection head is large, whereas FIG. 10 shows a case wherein the gap G is small.
In FIG. 9 and FIG. 10, the solid lines denote the waveforms of the analog signal A1 output through the midpoint 14a of the bridge circuit 14, whereas the chain lines denote the waveforms of the analog signal A2 output through the midpoint 14b.
Referring now to FIG. 4 through FIG. 10, the operation of a typical rotation detecting apparatus will be outlined.
As shown in FIG. 4 through FIG. 6, the permanent magnet 103 and the magnetic body 100 are disposed with the IC 104 between them; as the magnetic body 100 rotates, the projected sections 100a and the recessed sections 100b of the magnetic body 100 are alternately opposed to the permanent magnet 103.
Hence, the magnetic field applied to the IC 104 increases and decreases alternately as the magnetic body 100 rotates.
More specifically, the magnetic field applied to the IC 104 increases when the projected sections 100a face against the magnetic detection head because the permanent magnet 103 and the magnetic body 100 are closer to each other, whereas it decreases when the recessed sections 100b face against the magnetic detection head because the permanent magnet 103 and the magnetic body 100 are farther away from each other.
In this manner, the magnetic flux intersecting with the giant magnetoresistive element 20 on the IC 104 changes; therefore, the resistance values of the respective segments 20a through 20d vary as shown in FIG. 8 according to the direction and magnitude of the magnetic field applied.
Hence, the analog signals A1 and A2 as shown in FIG. 9 or FIG. 10 are generated at the output terminals, i.e., the midpoints 14a and 14b of the bridge circuit 14 shown in FIG. 7, as the magnetic body 100 rotates.
The fluctuating voltage levels of the analog signals A1 and A2 change according to the size of the gap G as shown in FIG. 9 and FIG. 10, and therefore, they change according primarily to the shapes of the magnetic body 100 and the rotation detecting apparatus 101 and the variations in the installed section, not shown, of the rotation detecting apparatus 101.
FIG. 11 shows the waveforms illustrating the operation for shaping the analog signal A received from the giant magnetoresistive element 20 into the pulse signal P.
As previously mentioned, since the giant magnetoresistive element 20 exhibits hysteresis as shown in FIG. 8, the analog signal A indicated by the solid line has a hill VH and a valley VL which are of relatively high levels and of extremely steep slopes, i.e., a high changing rate, corresponding to the ends of the projecting section 100a of the magnetic body 100.
Therefore, the binarized pulse signal P can be obtained by comparing the analog signal A with a reference voltage signal V14 indicated by the dashed line, by employing a well-known waveform processing circuit which will be discussed later.
The voltage level of the pulse signal P switches between high level and low level corresponding to the ends of the projected sections 100a.
FIG. 12 is a circuit diagram illustrative of a conventional waveform processing circuit described in, for example, Japanese Examined Patent Publication No. 5-70191; and FIG. 13 shows the waveforms of the voltage signals in FIG. 12.
In FIG. 12, an amplifier circuit 1 amplifies the analog signal A and outputs an analog voltage signal V12.
A level shifting circuit 4 connected to the output terminal of the amplifier circuit 1 is composed of resistors R12, R11, and R13 inserted in series between the output terminal of the amplifier circuit 1 and ground; it outputs a maximum level voltage signal V12 through one end of the resistor R12, a medium level voltage signal V11 through the junction of the resistors R12 and R11; and a minimum level voltage signal V13 through the junction of the resistors R11 and R13.
The voltage signal V11 is set so that it is lower than the voltage signal V12 issued from the amplifier circuit 1 by a predetermined voltage which is determined by the resistance ratio of the resistors R11 through R13; and the voltage signal V13 is set so that it is lower than the voltage signal V11 by a predetermined voltage which is determined by the resistance ratio of the resistors R11 through R13.
A voltage retaining circuit 5, which generates the reference voltage signal V14 according to the voltage signals V12 and V13, is equipped with a comparator 22 that retains the minimum value of the maximum level voltage signal V12, a comparator 23 that retains the maximum value of the minimum level voltage signal V13, diodes D2 and D3 inserted to the output terminals of the comparators 22 and 23, respectively, and a capacitor C inserted between the ends of the diodes D2 and D3 and ground.
The voltage signal V12 is applied to a non-inverting input terminal (+) of the comparator 22, the output terminal of the comparator 22 is connected to the cathode of the diode D2, and the anode of the diode D2 is connected to one end of the capacitor C and an inverting input terminal (-) of the comparator 22.
The voltage signal V13 is applied to a non-inverting input terminal (+) of the comparator 23, the output terminal of the comparator 23 is connected to the anode of the diode D3, and the cathode of the diode D3 is connected to one end of the capacitor C and an inverting input terminal (-) of the comparator 23.
One end of the capacitor C alternately retains the minimum value of the voltage signal V12 and the maximum value of the voltage signal V13 and output them as the reference voltage signal V14.
A comparator circuit 6 compares the medium level voltage signal V11 with the reference voltage signal V14 to output the pulse signal P obtained by binarizing the voltage signal V11.
At this point, a hysteresis generating circuit 10 provided at the input terminal of the comparator circuit 6 adds to the voltage signal V11 and applies the result, as the voltage signal V11b, to the non-inverting input terminal (+) of the comparator circuit 6.
The hysteresis generating circuit 10 is constituted by a resistor inserted between the voltage signal V11 and the non-inverting input terminal (+) of the comparator circuit 6, and by a resistor inserted between the output terminal and the non-inversion input terminal (+) of the comparator circuit 6.
A starting circuit 25 connected to the non-inverting input terminal (+) of the comparator circuit 6 prohibits the operation of the comparator circuit 6 for a predetermined period of time immediately after start-up.
The conventional waveform processing circuit shown in FIG. 12 alternately retains the minimum values of the voltage signal V12 and the maximum values of the voltage signal V13 to provide the reference voltage signal V14 indicated by the dashed line in FIG. 13.
In the comparator 6, the pulse signal P is obtained, the level of which is switched each time the voltage signal V11b intersects with the reference voltage signal V14.
However, when the level shifting circuit 4 is composed only of the resistors R11 through R13, the predetermined voltage shifting amount among the respective voltage signals V11 through V13 unavoidably varies according to the level of the analog signals issued from the amplifier circuit 1.
More specifically, as shown in FIG. 13, when the amplitude level of the analog signal is relatively high, an area wherein the voltage shift amount is smaller is generated in the vicinity of zero volt, the area being an area B indicated by the chain line, shows very small intervals among the voltage signals V11b, V12, and V13. This leads to smaller voltage differences of the voltage signal V11b and the reference voltage signal V14 between the input terminals of the comparator circuit 6.
FIG. 14 shows a waveform diagram of an enlarged view of area B indicated by the chain line shown in FIG. 13; it illustrates the waveform observed during a period in which the minimum value of the voltage signal V12 (the pulse signal P is at the low level) is retained as the reference voltage signal V14.
As shown in FIG. 14, when the voltage difference between the voltage signal V11b and the reference voltage signal V14 is small, if a noise signal VN that exceeds the voltage difference is superimposed, then the comparator circuit 6 inevitably outputs a pulse signal P that includes a malfunction signal PN based on the noise signal VN.
FIG. 15 shows the waveform observed when the amplitude level of the analog signal output from the amplifier circuit 1 is low; it illustrates the waveform observed during a period in which the maximum value of the voltage signal V13 (the pulse signal P is at the high level) is retained as the reference voltage signal V14.
In this case, the amplitude level of the analog signal, i.e., the voltage signal V11b, is lower than the voltage shift amount.
As shown in FIG. 15, if the amplitude level of the voltage signal V11b is low in relation to the voltage shift between the voltage signal V11b and the reference voltage signal V14, then the voltage difference between the input terminals of the comparator circuit 6 increases accordingly. As a result, the voltage signal V11b no longer intersects with the reference voltage signal V14, indicated by the dashed line, making it impossible to drop the pulse signal P from the comparator circuit 6 down to the low level.
When the hysteresis generating circuit 10 is constructed only by the resistors as shown in FIG. 12, the hysteresis width added to the voltage signal V11b depends on the voltage signal V11 and the level of the pulse signal P, and it is determined by the resistance voltage ratio of the voltage signal V11 and the pulse signal P.
However, the pulse signal P which has been subjected to waveform processing has a fixed value of either the high or low level, whereas the levels of the analog signal A and the voltage signal V11 change according to various conditions; therefore, the hysteresis width, which depends on the resistance voltage ratio, also changes.
If the hysteresis width of the voltage signal V11b supplied to the comparator circuit 6 varies, then there is a danger that the comparator circuit 6 may output the malfunction signal as illustrated in FIG. 14 or prevent the waveform processing from being implemented as shown in FIG. 15.
Further, as shown in FIG. 12, leakage current I4 flows from the inverting input terminal (-) of the comparator circuit 6 to the capacitor C; therefore, the reference voltage signal V14 increases with time when the voltage retaining state remains unchanged.
FIG. 16 is a waveform diagram illustrating the reference voltage signal V14 in the zone wherein the maximum value of the voltage signal V13 is retained.
In FIG. 16, the reference voltage signal V14, which is indicated by the dashed line, from the voltage retaining circuit 5 continues to rise due to the influences of the leakage current I4 even in the constant-level zone of the voltage signal V13.
The influences of the leakage current I4 cannot be ignored when the frequency of the analog signal A is low, and the voltage retaining circuit 5 cannot retain an accurate reference voltage signal V14, resulting in the malfunction of the comparator circuit 6.
Thus, in the conventional magnetic detecting apparatus, the level shifting circuit 4 is constituted only by the resistors R11 through R13 and the voltage shifting amount varies according to the level of the analog signal. Hence, when the amplitude of the analog signal is large as illustrated in FIG. 13, if the noise signal VN (see FIG. 14) is superimposed in the area B wherein the voltage shifting amount is small, then the problem arises in that the comparator circuit 6 issues the malfunction signal PN.
Further, as shown in FIG. 15, when the level of the analog signal is high and the amplitude thereof is small, the voltage shifting amount increases, presenting a problem in that the comparator circuit 6 cannot carry out the waveform processing.
There has been another problem: since the hysteresis generating circuit 10 is composed only of the resistors, the hysteresis width added to the voltage signal V11b varies according to diverse conditions, causing the comparator circuit 6 to malfunction.
There has been still another problem: when subjecting the analog signal A of a low frequency to the waveform processing, the reference voltage signal V14 of the voltage retaining circuit 5 inevitably changes due to the leakage current I4 from the comparator circuit 6 as illustrated in FIG. 16, preventing an accurate pulse signal P from being generated.